Determination of the nitrogen isotope enrichment factor associated with ammonification and nitrification in unsaturated soil at different temperatures

Abstract

For nitrogen (N) migration and transformation from unsaturated soil to groundwater, the N stable isotope (δ¹⁵N) was modified due to the isotope fractionation effect. To quantitatively evaluate the N cycle in groundwater systems, the determination of isotope fractionation is decisive. In this research, for the first time, incubation experiments were conducted to quantitatively investigate the N isotope enrichment factor (ϵ_p/s) associated with ammonification in unsaturated soil. Under weak isotopic fractionation, the Rayleigh function cannot be directly applied during ammonification. Thus, we proposed a different method calculating the ϵ_p/s values during ammonification, which were -0.03‰ for 15 °C and -2.34‰ for 30 °C. Moreover, for the first time, experimental equipment is presented to explore the isotopic fractionation effects under the co-occurrence of nitrification and volatilization. The results indicated that the isotope effect of volatilization during nitrification can be ignored in this study, and the ϵ_p/s values during nitrification were -10.59 and -6.81‰ at 15 and 30 °C, respectively. This work provides a novel arrangement determining the crucial parameters for identifying nitrate pollution sources in groundwater systems.

Keywords: Ammonification; Isotopic enrichment factor; Nitrification; Nitrogen isotope; Soil temperature.